1. Field of the Invention
The present invention relates to an image forming apparatus, such as an electrophotographic apparatus, which forms a color image by overlapping a plurality of color toners, and to an image forming method.
2. Description of the Related Art
In a fixing device of a conventional electrophotographic image forming apparatus, a fixation temperature range for good fixation on thick paper is narrower than a fixation temperature range for good fixation on ordinary paper.
In addition, while paper is being passed through the fixing device, a temperature variation range of a heat roller is greater when thick paper is passed than when ordinary paper is passed. Consequently, a low-temperature-side offset tends to easily occur. Jpn. Pat. Appln. KOKAI Publication No. 09-304979, for instance, discloses a structure for varying image formation conditions depending on characteristics of paper.
As described above, in the fixing device of the electrophotographic image forming apparatus, the fixation temperature range for good fixation on thick paper is narrower than that for good fixation on ordinary paper. In addition, the temperature variation range of the heat roller is greater when thick paper is passed than when ordinary paper is passed. Consequently, a low-temperature-side offset tends to easily occur. Moreover, when thick paper is used, the temperature control for the fixing device is difficult, and a great deal of time is needed to achieve a desired uniform temperature. As a result, the productivity for copying considerably deteriorates.